refactors automation auth code

[ChrisC]

In prep for the upcoming automation repo migrations over to w3c, I took this opportunity to update the GH app auth code to:

• continue moving hard-coded ID's into env variables, including the GH app and app install ID's as well as the GH workflow repo names (this supersedes uses env vars for gh app variables #1663)
• removes the need to store a bare jwt-signing-key on a local machine by adding it as another env variable
• uses octokit for simplified JWT handling